1. Field of the Invention
The present invention relates to an electrophotographic printer and, in particular, to an image formation device for forming an image signal comprising pulse signals each determining a laser pulse width, based on digital multivalued pixel data.
2. Description of the Related Prior Art
In an electrophotographic printer, a latent image is electrostatically formed on a photoreceptor scanned by a laser beam switching on and off according to image signals in pixels, and a visible image is developed by applying toner to the electrostatically charged photoreceptor. Exposure time and toner concentration of the photoreceptor have a relationship such that toner concentration gradually varies with exposure time within the range L as shown in FIG. 1. Therefore, halftones can be achieved by controlling the exposure time, or laser pulse width, within the region L.
A conventional image formation device is designed to operate in the range L. Specifically, on receipt of input digital data indicating gradations for each pixel, the conventional device generates an image signal comprising pixel pulse signals each having a pulse width corresponding to duration time of the laser beam which is varied in the range L of FIG. 1.
In the range L, however, toner attracted to the electrostatically charged areas on the photoreceptor is scattered rather than concentrated. Thus, when images containing photographs and characters or figures are printed, photographs are successfully transformed to printed halftone images but the characters and figures fail to provide clear outlines.
For binarization images such as characters or figures, there has been a method of improving resolution in the primary scanning direction to smooth the outlines thereof. In such a smoothing technique, the pulse width of a laser beam for an objective pixel must be controlled finely using data of surrounding pixels to improve resolution. However, the smoothing technique cannot be applicable to the conventional image formation device because it uses all input data as gradation data as shown in the range L of FIG. 1.
It is an object of the present invention to provide an image formation device that can provide clear outlines of characters and figures while maintaining the gradation of photographs.
It is another object of the present invention to provide an image formation device that enables the smoothing technique to be applied to binarization images.